Electrical prospecting



Jan; 19, 1932.

C. R. NICHOLS ET AL ELECTRICAL PROSPECTING Original Filed Aug. 20, 1927INVENTORS a (HA/Q4 5 400104 5 SAW/0:2 MA 4 A5 TOA/ 4 7 '1 ATTORNEYPatented Jan. 19, 1932 UNITED STATES PATENT OFFICE CHARLES R. NICHOLSAND SAMUEL H. WILLISTON, 0F DALLAS, TEXAS ELECTRICAL PROSPECTINGApplication filed August 20, 1927, Serial No. 214,234. Renewed December10, 1,931.

The general object of the present invention is to provide an improvedmethod of, and apparatus for determining the location and/ or characterof sub-surface bodies or earth portions difi'ering in electricalresistance from adjacent earth portions.

More specifically, the object of the invention is to provide an improvedmethod of, and means for determining the effects of an electricalenergization of separated earth points, on potential or current valuesat another earth point, conveniently referred to as the explorationpoint, and located at a distance from the points of energization.

The present invention is characterized by the provisions made for theeffective use of the portion of the energizing circuit or circuit systemwhich is external to the earth itself, to establish a definite or basepotential at a point in said circuit system which may be utilized indetermining the potential at the exploration point, or in makingmeasurements dependent on the diflerence between said base potential andthe exploring electrode potential.

In a preferred practical mode of using the invention, an exploringelectrode in contact with the earth at an exploration point is connectedto the base potential point in the energizing circuit system by acircuit portion, including a suitable instrument, such as a galvanometeror sensitive ammeter, for measuring potential differences between theelectrode and base potential point, or for measuring current flows whichresult from such diflerences in potential. For example, the exploringelectrode may be moved over the earths surface until a point or pointson the latter are located which have the same 0- tential value as saidbase potential point. The location of the equipotential point or pointsso determined, relative to some geometric point or points fixed by thelocation of the energized points, furnishes evidence of the characterand location of disturbing bodies beneath the surface of the explorationfield of different resistance from the main earth material beneath saidfield. In analternative use of the invention, the exploring elec-' lylocated with respect to the energization points, and the character andlocation of disturbing sub-surface earth ortions are determined by thecharacter of the current flow, if any, between the exploring electrodeand the base potential point.

The present invention is especially characterized by the manner in whichthe base potential point in the energizing circuit is established, andin particular by the steps taken to compensate for the variable andordinarily unknown, contact resistance between the earth and theexternal energization circuit at each point of energization.

Those skilled in the art will understand that the term point ofenergization as used herein, does not mean a single mathematical pointat which the external energization circuit is in contact with the earth,but ordinarily represents a central point about which, or a line alongwhich, a plurality of electrodes connected to the external energizingcircuit, are in contact with the earth. It is possible in some specialcases to make contact with the earth at each energization point by meansof a single contact or electrode, but in such case the single contactelectrode employed must be of considerable size to minimize the earthcontact resistance, which ordinarily can best be minimized by means of aconsiderable number of such contacts at each energization point. Whethera single large electrode or a number of small electrodes be employed,the resistance between the earth and the external energizing circuitwill ordinarily be different between different portions of the contactmetal and the adjacent earth. These differences are due to such causesas moisture, compactness, and other physical, as well as chemicalcharacteristics of the particular portions of the earth in immediatecontact with, or closely adjacent the energization contact electrodes.

In accordance with the present invention, adjustable resistances are sodisposed and adjusted in the external energization circuit as toeliminate, so far as the location of the base potential point isconcerned, any and all effect due to the unknown earth contactresistance- In practice, the adjustable resistances are so located inthe separate branches of the energization circuit connected to thedifferent energization points, that the resistances of the differentbranches may be made equal, or in definite proportion to one another.This means in practice that the sum of the adjustable resistance in eachbranch of the energizing circuit, and the unknown earth contactresistance at the energized point to which the branch is connected, ismade equal to or in definite proportion to the sum of the externalresistance in, and the earth contact resistance pertaining to each ofthe other energizing circuit branches.

The various features of novelty which characterize our invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,however, and the advantages possessed by it, reference should be had tothe accompanying drawings and descriptive matter in which we haveillustrated and described preferred embodiments of the invention.

Of the drawings Figs. 1 and 2 are diagrams illustrating circuitarrangements successively employed in one mode of operation;

Figs. 3 and 4 are diagrams illustrating a second mode of racticing theinvention; and

Fig. 5 is a diagram illustrating a third mode of procedure.

In Fig. 1, A and B represent electrodes in contact with the earth atseparated energization points, and each connected to one terminal E of asource of current E which may be an alternator, but preferably is astorage battery or other suitable source of direct current. As shown,the second terminal E of the current source E is connected to anelectrode C in contact with the earth at a substantial distance fromeach of the electrodes A and B. The connections between the terminal Eand the electrodes A and B include resistances R and B, respectively.One of these resistances must be adjustable, and as shown, each isadjustable. Advantageously a resistance 1", is interposed between theterminal E and the resistance R, and a resistance r equal to theresistance r is interposed between the terminal E and resistance B. Asshown, a'galvanometer F connects the terminals of the resistances 1" and7' remote from the terminal E.

In the contemplated use of the apparatus shown in Fig. 1, the relativevalues of the resistances R and R in circuit with the electrodes A andB, respectively, are varied until the galvanometer F indicates thatthere is no potential difference between its terminals. When thisoccurs, the sum of the unknown contact resistance between the earth andelectrode A and the resistance R, will be practically equal to the sumof the resistance R plus the unknown contact resistance between theelectrode B and the earth, and the current flowing throu h each of theelectrodes A and B will then %e alike.

The above mentioned practical resistance equality results from the knownfact that the earth resistance between the electrodes A and C andbetween the electrodes B and C is confined almost entirely to what isherein .referred to as contact resistance. The cross section of theconductor formed by the earth is so large that any difference in earthresistance not due to the character of the contact of electrodes A, Band C with the earth and the composition of the earth within acomparatively few feet of the electrodes proper, is negligible.

A sub-surface body along the line AC, for example, differing inconductivity from the general earth conductivity may result in readilymeasurable effects on the potential difference between separated pointsof the earth surface adjacent the line AC- without having anysignificant influence in the total earth resistance between theelectrodes A and C.

After the total resistances of the two branches of the energizationcircuit are thus balanced, and without change in adjustment of theresistances R and R, the circuit arrangement shown in Fig. 2 isestablished. In changing from the arrangement shown in Fig. 1 to thatshown in Fig. 2, the circuit branch including the resistance r isdisconnected from the terminal E, and connected to the terminal E, fromwhich the electrode C is disconnected. In Fig. 2 a high potentiometerresistance R is connected between the terminals of the resistances R andR adj acent the current source E, and an exploring electrode G isconnected through an ammeter or galvanometer FA to the resistance R atan intermediate point of the latter so located that the resistances ofthe portions of the resistance R at each side of said point are in adefinite predetermined ratio, which in many cases is advantageouslyunity.

With the circuit arrangement shown in Fig. 2, and with the resistancesof the two energizing circuit branches balanced, as described, thegalvano 'meter or sensitive ammeter FA connected in circuit with theelectrode G, will give useful and readily interpreted indications ofthe-earths structure beneath the exploration field. If the fixedpotential point in the potentiometer R to whlch the exploring electrodeG is connected lies between sections of the resistance R of equal value,the instrument FA may be used to locate the mid-potential line on theearth surface between the electrodes A and B, by moving the exploringcontact G around on the earth surface until a suitable number of pointsalong said mid-potential line are located. Whenthe electrode G is incontact with the earth along said line the instrument FA will give azero reading, provided the effect of stray earth currents, if any, isneutralized. Alternatively, if the ex loring electrode G is located onthe line between the points A and B at a point midway between thelatter, or at some other point definitely located geographically withrespect to said points A and B, the indications of the instrument FA mayadord useful indications of the nature of the mid-potential linedisplacement from the point/at which the exploring electrode G islocated.

In the circuit arrangement illustrated in Fig. 3, the energizationpoints A and C are connected to one side of the source of current Ethrough separate circuit portions exactly like those shown in Fig. l,and the energization points B and D are similarly connected to theopposite side of the source oi: current. The points B and D aresymmetrically disposed with respect to the points A and C, and atopposite sides of the line A-C. The energization circuit branch runningto the point B includes a fixed resistance r and an adjustableresistance R, and the branch running to the point D includes a fixedresistance r and an adjustable resistance R In Fig. 3, the potentiometerre sistance R is connected to the energization branch for the point C,between the resistance r and adjustable resistance B and to theenergization branch for the point D at a point between the resistance Rand the resistance r in that branch.

It will be apparent that when the resistances R, R R and R are sorelatively adjusted that the total resistance including the unknownearth contact resistance in the separate energization circuit branchesare all alike, the mid-point in the potentiometer resistance R wil be atthe same potential as v the mid-potential earth surface point in theexploration field. The last mentioned point will be the intersectionpoint of a mid-potential line passing between A and D and between C andB, and a mid-potential line passing between A and B and between C and D.With the energization points at the corners of a square as shown, or atthe corners of a rcctangle, t-he point of intersection of saidmid-potential lines will coincide with the geometric center of saidsquare or rectangle, provided the resistance of the earth beneath theexploration field is uniform. If said resistance is not uniform, themid-potential point of the exploration field will be displaced from saidgeometric point in a direction and to an extent which can be determinedor accurately estimated by means of the exploring contact G connected tothe mid-potential point of the potentiometer resistance R, by followingthe procedure previously described in connection with Figs. 1 and 2.

With the arrangement shown in Fig. 3,'a proper relative adjustment ofthe resistances R and R is insured when the instrument F indicates nopotential difiference as a result of the currents flowing from thegenerator through the corresponding resistances r and a. While theinstrument F shown in Fig. 3, forms a means for determining when currentthrough the energization branches running to the energization points Band D are equal, the readings of the instruments F and F in thearrangement shown in Fig. 3 are not sufficient to determine the relativeadjustments of the resistances R, R R and R required. Such relativeadjustments may be secured, however, by iirst interchanging theconnections between the generator E and the points D and C as indicatedin Fig. a so as to reverse the polarity of those points, and then,without change in the resistances R and R adjusting the resistances Rand R until the instruments F and F indicate zero potential difierencesbetween their terminals. When this has been done, the resistances in allfour energization branches will be the same provided, of course, that inthe meantime there has been no change in the resistances of the circuitbranches running to the points A and C.

After the resistances in the four separate energization branches arethus balanced, the circuit arrangement of Fig. 3 is again establishedand by means of the electrode G and instrument F, the mid-potentialpoint of the exploration field may then be located, or the current flowdue to the potential diflerence between the geometric center of thefield and the mid-potential point of the potentiometer resistance B maybe measured.

An alternative method of balancing the resistances in four energizationcircuit branches of the kind shown in Fig. 3, is illustrated in Fig. 5.In Fig. 5 the four separate branches of the energization circuit are allconnected to one terminal E of the source of current E, and the otherterminal E of the source of current is grounded, as by means of theexploring contact G. As shown in Fig. 5 a galvanometer F is connectedbetween the circuit energization branches for the points A and B.Another galvanometer F is connected between the energization branchesfor the points B and C, and a third galvanometer F is connected betweenthe energization branches for the points C and D, the connection to eachbranch being made at a point between the fixed resistance 1" or T etc,and the corresponding adjustable resistances R or R etc. in the branch.As will be readily apparent, when the resistances R, R R and R are soad'usted that the terminals of each of the di erent galvanometers F, F,F are at the same potential, the total resistance in each of'thedifferent energization branches will be the same. The circuitconnections shown in Fig. 5 are then shifted to establish the circuitarrangement illustrated in Fig. 3.

In practice each of the detectin circuits including the exploringelectrode illustrated in the different figures, may well includeamplifying provisions and provisions for eliminating the effect of strayearth currents such as are disclosed and claimed in our priorapplication Serial No. 129,430, filed August 16, 1926, and in practice,also, the energization currents su plied by the source E mayadvantageously e interrupted and reversed at regular intervals asdescribed in our said prior application. The features just mentioned ofthe invention disclosed in our rior application need not be described.in etail herein, however, as they form no part of the present invention,although they may advantageously, in some cases, at least, be used inpracticing the resent invention.

While in accor ance with the provisions of the statutes, we haveillustrated and described the best form of embodiment of our inventionnow known to us, it will be apparent to those skilled in the art thatchanges may be made in'the form of the apparatus disclosed withoutdeparting from t e spirit of our invention as set forth in the appendedclaims and that in some cases certain features of our invention may beused to advantage without a corresponding use of other features.

Having now described our invention, what we claim as new and desire tosecure by Letters Patent is:

1. In the method of electrical prospecting which comprises theenergization of spaced apart electrodes in contact with the earth,through separate energizing circuit branches connected to the differentelectrodes, the improvement which consists in giving said branches suchrelative resistance values as to compensate for the differences in theearth contact resistances of the corresponding electrodes.

2. In the method of electrical prospecting, the steps which consist inconnectin a plurality of spaced apart earth portions to separateenergizing circuit branches, and adj usting the relative resistances ofsaid branches so that the same current flow will he produced in eachwhen the same potential, different from the normal earth potential, isimpressed on each branch.

3. In the method of electrical prospecting the steps which consist inconnecting a plurality of spaced apart earth portions to a common sourceof potential through separate circuit branches includin electrodes incontact with said earth portions, adjusting the relative resistances ofsaid branches to obtain equal current flows in the different branches,and then connecting a source of potential difference between saidbranches,

tact with said earth portions, adjustin the relative resistances of saidbranches to o tain equal current flows in the different branches andthen connecting a source of potential difference between said branchesand establishing a base potential point at which the potential isintermediate that of the terminals of said source.

5. In the method of electrical prospecting the steps which consist inadjusting the relative resistances of separate energizlng circuitbranches connected to spaced apartearth 1portime so that the samecurrent flow wi produced in each branch when the same potential,different from that of the earth, is impressed on each branch, and thenafter so balancing the resistances of said circuit branches, impressinga potential difference on said earth portions by connecting the latterthrough said branches to a source of otential difference, and observingan e ect produced by the resultant current flow through the earth.

6. pparatus for electrical prospecting com rising electrodes in contactwith the cart at separate energization points, an energizing circuitincluding s e p a r a t e branches connected to the electrodes at thedifi'erent energization points, resistances in said branches adjustableto make the sum of such resistance and the earth contact resistance ofthe corresponding electrode bear a definite relation to t ecorresponding resistance sum for each .of the other branches, and anearth engaging electrode connected to said circuit at a base potentialpoint in said circuit between which point and the earth at each of saidenergization points definitely related potential diiferences exist.

7. Apparatus for electrical prospecting comprising electrodes in contactwith separated portions of the earth, a source of current, conductorsconnecting the opposing terminals of said source to differentelectrodes, a potentiometer resistance connecting the opposing terminalsof said source, adjustable resistances for equalizing the resistances ofthe difi'erent circuit branches each formed by a difi'erent electrodeand the corresponding conductor connecting it to the source of currentso as to compensate for varying earth contact resistances of thediflerent electrodes and an earth contact connected to an intermediatepoint in said potentiometer resistance. r

8. In the method of electrical prospecting which comprises theconnection of two separated portions of the earths surface to anexternal energizing circuit and thereby creating a potential differencebetween said portions, the improvement which consists in ad- 'ustingresistances in said external circuit to thereby create predeterminedrelative potential differences between said portions and a common pointin said energizin circuit and comparing the potential at sai point withthe potential of a third portion of the earths surface.

Signed at Dallas, in the county of Dallas,

and State of Texas, this sixteenth day of August, A. D., 1927.

CHARLES R. NICHOLS. SAMUEL H. WILLISTON.

